1. Field of the Invention
The invention relates to an actuator which includes a quick release unit on a transmission between an activation element and a reversible motor.
2. The Prior Art
The description of the invention departs from a linear actuator, comprising an electric motor which operates a spindle through a transmission. On the spindle there is a spindle nut with a piece of tubing attached at one end; this serves as the activation element. The other end of the piece of tubing is attached to the movable element in the construction where the actuator is built-in, while the other part of the actuator is attached with a rear fixture to the stationary part of the construction. By keeping the spindle nut from rotating, the nut is moved either outwards or inwards on the spindle, depending on the direction of rotation, and the piece of tubing is either moved axially outward or retracts in a manner similar to a piston rod in a telescopic cylinder.
The registration of the position of the tubing is typically performed by counting the number of revolutions made by the spindle from its initial position. The number of revolutions multiplied by the thread pitch determines the position of the tubing. An absolute determination of the position can be made using a potentiometer operated by the motor through a gearing. When the transmission ratios are known, the revolutions of the spindle can be calculated. A more common way of determining the position is by use of a magnetic or optical decoder. A magnetic encoder comprises a magnetic ring with a number of poles which activate a read-switch or Hall-element each time one of the poles passes the switch, thereby producing an electrical signal. Typically, with four poles, the position is determined by each quarter revolution of the spindle. An optical encoder works in the same way using a rotating disc with perforations and a light source. When a perforation passes the light source, light passes through to a photocell producing an electrical signal. Based on the signals, the position is calculated by a microprocessor in the control.
A quick release in an actuator is used for temporary release of the activation element for manual setting. An example of use of the quick release is in hospital beds, fire doors and other constructions where it is important or even essential to be able to spontaneously set the activation element.
Actuators with quick release function are known from, amongst others, EP 685 662 B1, WO 03/033946 A1 and WO 03/033946 A1 and EP 577 541 B1 all in the name of Linak A/S. The first three documents relates to a quick release construction based on a releasable clutch spring around a cylinder part on the activation element and a matching cylinder element from the exit of the motor/transmission. The last document EP 577 541 relates to a quick release construction using an angular gear where the pinion from the output of the motor/transmission may be displaced to release the grip with the pinion on the activation element.
A problem with the known actuators with quick release function is that the position determination is lost at release of the quick release, and it is necessary to reset the system. This is not normally a problem as the system is set up to reset the actuator by driving it to an outer position, and release of the quick release usually results in the actuator being brought to an outer position.
In some situations it may also be desirable to know the position after the quick release has been activated without the need to bring the actuator to an end position.